The present invention relates to an image forming apparatus of an electrophotographic method and more particularly to improvement of a developing device.
In recent years, there is an increasing need for speedup and high image quality for an image forming apparatus of an electrophotographic method. To satisfy the need of speedup, it is necessary to increase the linear speed of a latent image bearing member such as a photosensitive drum, though when the linear speed of the latent image bearing member is increased, various problems arise at the developing step.
At the developing step, in the problems relating to the image quality, a phenomenon that when the linear speed of the latent image bearing member is increased, the width of a fog margin called an operation window, which can be set, is decreased is a main problem.
The fog margin is a potential difference between the charging potential (unexposed part potential of the photosensitive drum) of the latent image bearing member and the DC component of the developing bias potential and the concerned fog margin is generally set to a value of causing neither fog nor carrier adhesion.
And, there are forward development for executing development by moving the latent image bearing member and developer bearing member in the same direction in the developing area where both bearing members are arranged opposite to each other and reverse development for executing development by moving the bearing members in the opposite directions and to widen the operation window of the fog margin and satisfactorily suppress an occurrence of fog and an occurrence of carrier adhesion, the reverse development is advantageous.
Further, in the high-speed development, to ensure the developing performance, the development is executed in the alternating electric field.
As a means for realizing a high-speed image forming apparatus like this, at the developing step, it is advantageous to use the backward developing method and execute development in the alternating electric field. However, even if the means is adopted, it is found that in the high-speed development, worm-shaped image irregularities are easily formed on an image.
Such worm-shaped image irregularities, as shown in FIG. 1, are uneven density generated in a solid image at low density and as a result of pursuit of the cause, it is considered that the uneven density is caused by the following phenomenon.
In the reverse development, the relative speed between the latent image bearing member and the developer bearing member is high, so that the friction between the magnetic brush and the latent image bearing member is increased, thus the latent image bearing member is easily charged by friction, and the latent image bearing member and developer bearing member leave the developing area, and when the gap between the two is expanded, discharge may be caused.
Worm-shaped image irregularities are inferred to be caused by such frictional charge and discharge.
To solve the problem of the image quality at the developing step, many researches have been made until now and many improvements have been proposed.
For example, in Patent Document 1, the linear speed ratio between the latent image bearing member and the developer bearing member is selected, and the intensity of magnetization of the magnetic carrier is made appropriate, thus missing of dot images is prevented.
Further, in Patent Document 2, the relationship between the AC component (Vpp) of the developing bias voltage and the development gap Ds for the difference between the unexposed part potential of the photosensitive drum and the exposure part potential is set appropriately, thus carrier adhesion is prevented.
[Patent Document 1]                Tokkaihei 09-127793 (Japanese Non-Examined Patent Publication)        
[Patent Document 2]                Tokkai 2001-5295 (Japanese Non-Examined Patent Publication)        
For worm-shaped image irregularities generated in the high-speed development for moving the latent image bearing member such as the photosensitive drum at a high speed, the conventional arts including Patent Documents 1 and 2 explained above are not sufficient measures.